The subject matter disclosed herein relates generally to motors and, more specifically, to systems for use in controlling the output torque of electric motors and methods of assembling such systems.
At least some known electric vehicles include a battery-type power supply, a power converter coupled to the battery, an electric drive motor coupled to the power converter, and a load coupled to the motor via a drive shaft. These known electric vehicles also include a control system that facilitates operation of the power converter to transform the direct current (DC) supplied by the battery to alternating current (AC) having a variable frequency and a variable amplitude for transmission to the motor. The power converter causes the motor to induce a torque to the load, which also determines the electric power transmitted to the motor from the converter.
At least some of these known electric vehicle control systems use a limit on the output torque to limit the DC power transmission from the battery merely as a function of the remaining strength of the battery. Limiting the power draw from the battery is accomplished by limiting the torque output of the motor, thereby limiting motor power. One method of measuring the remaining strength of the battery includes measuring the actual terminal voltage of the battery. Therefore, traditionally, power limiting is accomplished by derating the converter output torque while operating under a reduced battery strength. Torque limiting also facilitates directly reducing current when thermal limits are being approached.
In at least some of these known electric vehicle control systems, such torque limiting unnecessarily limits performance when the derating is due to limited power availability. Such control scheme typically limits motor torque to induce a most restrictive drive shaft speed and unnecessarily limits the amount of torque available for braking or driving the load at low speeds. Moreover, rapid fluctuations of the battery terminal voltage during normal operation may induce similar fluctuations of the motor torque, thereby facilitating transient torque changes and drive shaft vibration.